Optical transmission, in which an information signal is modulated onto an optical carrier, is widely employed in modern communication systems. In particular, many long-haul transmission networks employ single-mode optical fibres for the transmission of digital information at high bit rates, using one or more optical carriers, or wavelengths, over each fibre. Indeed, recent advances in optical technologies, including improvements in optical fibres, optical modulators, and the development of reliable and commercially practical optical amplifiers, have enabled the deployment of optical transmission paths capable of transporting on the order of 1 Tb/s over distances of thousands of kilometres, without the need for electronic regeneration. Individual optical channels in such systems typically carry information streams at rates of 2.5 Gbit/s, 10 Gbit/s, or even higher.
However, long-haul optical transmission systems are ultimately limited by distortion and degradation of the transmitted signals, arising primarily from dispersion (e.g. chromatic and polarisation mode dispersion), non-linear transmission processes, and noise introduced by optical amplifiers. Various techniques are known, or have been proposed, for overcoming, or at least mitigating, these various sources of signal degradation. Dispersion compensation, in particular, has been a very active area of research and commercial development in recent years. It is well known, for example, that linear dispersion processes, including chromatic dispersion, can be reversed by suitable optical equalisation means. However, equalisation in the optical domain has a number of disadvantages or limitations. The most practical, and widely deployed, means for the optical compensation of chromatic dispersion is to propagate dispersion-affected signals through a length of dispersion compensating fibre (DCF) which has been selected to have a total dispersion that is substantially an inverse of the dispersion accumulated in the transmission fibre. However, DCF represents an additional signal propagation path having its own attenuation and non-linear properties, and accordingly there is a limit to the amount of dispersion that can be compensated in this manner while maintaining adequate overall signal quality. Furthermore, the length of DCF used must be accurately matched to the transmission path, and potentially also to the transmission wavelength, or range of transmission wavelengths, and accordingly systems employing DCF for dispersion compensation may have limited flexibility and/or reconfigurability. Similar problems arise with alternative fixed optical equalisation means, such as dispersive optical filters or grating-based devices, and indeed such components are typically even more strongly wavelength dependent than DCF.
In order to mitigate the abovementioned problems and limitations of optical dispersion compensation methods, there has recently been increasing interest in the development of electronic dispersion compensation techniques. Previously, it had not been considered viable to perform significant signal processing within the electronic domain, when operating at the very high bit rates employed in long-haul optical transmission systems. However, with recent technological advances and improvements in electronic and digital systems and devices, very high speed analog and/or digital signal processing has become a practical option. This has led to the proposal and development of systems and apparatus providing various degrees of dispersion compensation within the electronic domain.
In order to fully compensate within the electronic domain for dispersion arising within the optical transmission channel, it is generally necessary to preserve phase information across the interface between the optical and electrical domains, in either the transmitter, the receiver, or both. For example, pre-compensation techniques involve the generation of an optical signal at a transmitter having amplitude and phase characteristics calculated such that, following transmission through a dispersive channel, the resulting optical signal detected at the receiver is substantially free of distortion due to dispersion. Alternatively, post-compensation techniques require that a received signal, affected by dispersion in the optical transmission channel, is detected such that the optical amplitude and phase information is preserved into the electronic domain, to enable full equalisation to be performed.
Conventionally, optical transmitters have employed intensity modulation, while direct detection has been used at the receiver. Intensity modulation and direct detection (IMDD) systems are thus generally the simplest, least costly, and most well-understood optical transmission systems. However, neither intensity modulation (which produces an optical signal, the power of which is proportional to the modulating electrical signal amplitude) nor direct detection (which produces a received electrical signal, the amplitude of which is proportional to detected optical power) preserves either the optical phase, or optical field amplitude, across the optoelectronic interface. It is therefore generally believed that IMDD systems are incompatible with the use of electronic dispersion compensation techniques.
One particular consequence of dispersion in IMDD systems is the generation of deep nulls within the received signal spectrum. This effect is illustrated in FIG. 1, which shows the frequency response 100 of an IMDD system resulting solely from the effects of chromatic dispersion in the optical transmission path (i.e. neglecting the frequency response of the transmitter and receiver components). The intensity-modulated optical signal includes upper and lower spectral sidebands, each of which experiences a different optical phase shift due to chromatic dispersion, relative to the optical carrier. At the receiver, in the process of direct detection, the spectral components of each sideband respectively mix with the optical carrier to produce a baseband electrical signal. Each frequency component of the electrical signal results from a corresponding sum of contributions from the respective upper and lower sideband components. These may interfere either constructively, to produce a peak in the RF frequency response (e.g. 102), or destructively, to produce a corresponding trough, or null, in the response (e.g. 104). The nulls in the frequency response effectively represent unusable portions of the spectrum, since the signal-to-noise ratio at such frequencies will be extremely poor. Accordingly, no electronic compensation method can effectively recover a signal having substantial information content within the spectral nulls of the frequency response 100.
It is accordingly an object of the present invention to provide a receiver apparatus, and corresponding method, which is able to mitigate the aforementioned problems of the prior art, to enable improved recovery of information signals that have been transmitted over a dispersive optical channel, and which is compatible with the use of intensity modulation and direct detection.